Various types of clays are widely available around the world. Some types of clays are layered or smectic clays that can be subdivided into individual layers. Such clays have been used in an attempt to reinforce polymers. Through some combination of mechanical reinforcement and selected crystallization of thermodynamically disfavored forms of the polymers, significantly enhanced thermal and/or mechanical properties are obtained when compared to the unfilled starting polymers or their more traditionally filled composites. In most cases, however, clay reinforcement of polymers does not result in property enhancement. One reason for sub-par performance is that to achieve useful exfoliation of the clay, the favorable thermodynamics which led to the clay's formation in nature must be overcome, and few polymer systems can provide such a thermodynamic driving force.
In an attempt to provide materials with beneficial properties, inorganic materials such as silicas, clays, metal oxides and the like have been formed into aerogels.
The production of inorganic aerogels from silica and water was first reported by S. S. Kistler, see “Coherent Expanded Aerogels”, J. Phys. Chem. 1932; 36: 52-64. In this initial process, water was slowly removed from an aqueous silica mixture via a solvent exchange process using ethanol or ether to produce low density structures. The preparation of montmorillonite clay aerogels by freeze-drying clay hydrogels was reported by R. C. MacKenzie, see “Clay-water Relationships”, Nature 1952: 171: 681-3, and F. Call, “Preparation of Dry Clay-Gels by Freeze-Drying”, Nature 1953; 172: 126; the resultant fibrous montmorillonite structures were described as possessing reasonable rigidity, but poor thermal stability at 110° C. for extended time or when desiccated over phosphorus pentoxide. Similar processing of non-swelling clays, such as kaolin, only produced fine powders.
Weiss et al., see “The Skeleton Structure in Thixotropic Gels”, Naturwissenschaften 1952; 39: 351-2, and Hoffman et al., see “A Thixotropy in Kaolinite and Inner-Crystalline Swelling in Montmorillonite”, Kolloid-Z; 1957, 151, 97-115, studied several clay-solvent combinations that produced rapid setting, thixotropic gels. These authors demonstrated that high vacuum sublimation of frozen thixotropic clay gels in water or benezene produced “gel skeletons” with remarkable elasticity (elastic compression up to 75% of their original volumes). Montmorillonite interlayer spacings, measured by x-ray diffraction, were shown to inversely correlate thixotropic and sedimentation volumes. Weiss proposed that high thixotropic volumes of montmorillonite solutions containing alkali ions were caused by a splitting of the montmorillonite crystal into thin layers.
The effects of freeze-drying on the interlayer spacing of clay hydrogels was studied in greater depth by K. Norrish et al, see “Effect of Freezing on the Swelling of Clay Minerals”, Clay Miner, Bull. 1962; 5: 9-16. Although montmorillonite clay hydrogels were found to retain their shape and partial rigidity when water was removed the interlayer spacing in sodium montmorillonite decreased from greater than 30 to 10 Å during freezing. Upon thawing, the original clay morphology was restored. Ice crystal formation was thought to be responsible for the collapse of the swollen structure, thus defining a mechanism for structural transformation, but the structure of the gel itself remained elusive.
H. Van Olphen, “Polyelectrolyte Reinforced Aerogels of Clays-Application as Chromatographic Adsorbents”, Clay Miner, 1967; 15: 423-35, proposed that particles within clay aerogels, produced using a freeze-drying process, are linked edge-to-face much like a “house of cards” owing to opposite surface and edge charges that exist in clays. The author suggested ice crystals grow radially, pushing clay particles aside to promote parallel platelet alignment. The incorporation of polyelectrolytes into bentonite clay aerogels via freeze-drying of a polyelectrolyte-bentonite hydrogel was also studied by Van Olphen, who found the normally fragile aerogels to become stronger and tougher upon polyelectrolyte incorporation.
The effects of process parameters, such as clay concentration and freezing rates, upon the size and shape of resultant clay aerogels was investigated by Nakazawa et al., see “Texture Control of Clay-Aerogel Through the Crystallization Process of Ice”, Clay Sci. 1987; 6: 269-76, who reported that decreases in clay concentrations and freezing rates resulted in pore shape changes from polygonal cells to thin lenses. The authors proposed that pores remaining in the freeze-dried aerogel structure are “negatives” of the ice particles once formed and later sublimed. The aerogels produced by Nakazawa were reportedly stable to heat treatment up to 800° C., albeit with some shrinkage.
U.S. Pat. No. 2,093,454 to Kistler relates to reported improvements in the art and process of producing dry gels from colloidal solutions, and to the production of a gel, one continuous phase of which is a gas, and which Kistler defines as an aerogel. Kistler states that the aerogels are characterized by the fact that they are formed from colloidal gels in which the liquid menstruum is removed, at least in part, by heating the liquid under pressure beyond its critical temperature, and subsequently releasing the liquid thus heated. In this manner it is possible to produce a skeleton of the solid component of the gel as it actually exists before treatment and differing only in that the liquid medium is displaced by a vapor or gas.
U.S. Pat. No. 3,203,903 to Van Olphen et al. relates to inorganic aerogels characterized by reportedly substantial physical stability and to a process for preparing such aerogels. Van Olphen reportedly teaches a method for preparing physically stable, mechanically strong aerogels of inorganic materials. The method comprises forming a gel of the inorganic material in which there is uniformly and intimately admixed a polymeric material which has substantial solubility in the liquid which composes the dispersing medium of the gel, then removing the dispersing medium and replacing it with a non-condensable gas phase, under conditions such that no gas-liquid interface exists in the dispersing medium during its removal. The aerogels of inorganic materials prepared in this way reportedly have been found to have substantially greater mechanical strength than the aerogels of the same inorganic materials prepared by the methods known previously. It has reportedly been found, for example, that clay aerogels, which have little physical strength when prepared according to the prior art methods, when prepared according to the process of this invention can readily be formed into different shapes and are physically strong enough to retain those shapes when subjected to physical stresses encountered during their use. This invention reportedly provides inorganic aerogels of improved strength. Van Olphen only describes the use of a polymeric material that can be introduced into a precursor gel used to form an aerogel. Column 4, lines 15-33, state that the polymeric material can be first dissolved in a liquid to be used in forming the gel and then the gel formed, or the gel can be formed, then contacted with a solution of the polymeric material in that liquid, or in a different liquid. The inventors of the present application have found that the aerogel composites produced according to the teachings of Van Olphen are relatively fragile, and typically fall apart upon handling.
As such, there is a need in the art for aerogel materials having improved physical features over that of a clay aerogel whose mechanical properties are marginal.